Known electromagnetically shielded channel parts for forming a cable channel system in electrical installations are normally based on some electrically insulated thermoplastic material that is covered externally with a coating of electrically conductive metallic film, preferably by vacuum sputtering. The metallic film is, as a rule, connected electrically to earth with the intention of enhancing the shielding effect of the system comprised of said channel parts. Such channel parts are known from JP 513 72 for instance.
One drawback with these channel parts is that the metal coating on the plastic material tends to loosen as the channel parts are handled and during their transportation. In addition to the method being expensive in practice, it is also difficult when practicing the method to metallize the inner surfaces of the connection branches of junction boxes so effectively as to be able to guarantee the requisite electrical connection between the metal coating on the outside of, e.g, a tubular part of the system and the metallic coating on the inside of the connection branch pipe when the tubular part is received in the connection branch pipe. It is therefore necessary to arrange separate connection couplings between said parts in order to achieve the requisite electrical connection between the mutually connected parts of the system.
It has been found, however, that plastic materials that are conductive electrically shield alternating electrical fields very effectively, when earthed electrically. The plastic materials are normally made electrically conductivity admixing an electrically conductive material with the plastic, such as carbon fibre graphite, or carbon black for instance. Electromagnetically shielding compositions are well known to the art and described exhaustively in U.S. Pat. No. 5,004,561 for example.
The object of the present invention is to provide a system of electromagnetically shielding channel parts, including tubular parts, jointing or splicing sleeves, junction boxes having tubular connecting branches, for forming an electromagnetic wave- shielding channel system for electric cables, with which the aforesaid drawbacks are avoided and the channel parts can be produced more easily than the earlier known parts.
The publication U.S. Pat. No. 4,508,640 teaches electromagnetically shielding material that consists of several layers of mutually different electrical conductivity. Despite this document, similar to the aforegoing, giving an exhaustive report of the problems associated with electromagnetically shielding materials in the form of externally metallized thermoplastics for use in a cable channeling system the type of layered parts and components to which the present invention refers are neither mentioned nor inferred. In clear distinction to the principles on which the present invention is based, according to which an electrically insulating layer is applied to the inner surface of respective parts, the document proposes the application of an electrically insulating layer on parts and components manufactured from electrically conductive materials with the intention of enhancing safety when handling the parts.